Sound translating system



June 22, 1937. A. PFISTER SOUND TRANSLATING SYSTEM Original Filed July14. 1954 V 2 Sheets-Sheet 1 IAAIAAAA OIUIOIIO Reiaued June 22, 1931'PATENT orrica SOUND TRANSLATING SYSTEM i Arthur Pflster, Queens NewtoneEngineering Inc., New York, a corporation of New York Village, N. Y., assign'or to N. Y.,

Original No. 2,052,110, dated August 25, 1936, Se-

rial No. 735,190, July reissue April 8, 1937,

29 Claims.

The system may likewise include suitable tone I control devices arrangedto produce different ltransrnission characteristics for differentfrequency registers. The apparent quality of the sound, on reproduction,may thus be improved.

According to a preferred form of the invention as applied to a soundreproducing system a portion'of the sound energy 'may be passed throughan amplifier and a frequency selective circuit and fed to a sound outputdevice. Another portion of the sound energy may be passed through acontrol circuit where it may be averaged and utilized to control thegain ratio of the'amplifier. Control is eifected preferably by varyingthe potential applied to a separate control electrode situated in thespace discharge path of said amplifier.

Limiting devices may be included in the control circuit to determine thelimits to'which the control potential may be varied without causingdistortion of the selection. The control channel may likewise preferablyinclude sound frequency suppression elements to prevent soundfrequencies from reaching the control electrodes and causing distortionof the selection.

Adjustable delay means may also be included in the control channelwhereby the rate of change in gain ratio, on crescendo and decrescendo,may be adjusted according to the characteristics of the selection,whether it be speech or music, to render its reproduction'more pleasing.

Other refinements of the present system, whereby its operatingcharacteristics are improved and the translation of the sound selectionis rendered more perfect, are more fully brought out in the followingdetailed description.

The invention also consists in certain new and original features ofconstruction and combinations of parts hereinafter set forth andclaimed.

Although the novel features which are believed to be characteristic ofthis invention will be particularly pointed out in the claims appendedhereto, the invention itself, as to its objects and advantages, and themanner in which it may be car- 14, 1934. Application for Serial No.135,702

ried out, may be better understood by referring.

to the following description taken in connection with the accompanyingdrawings forming a part thereof, in which Fig. 1 shows an embodiment ofthe invention in a sound reproducing system;

Fig. 2 is a diagram illustrating action;

Fig. 3 is a diagram illustrating the controllable delay on crescendo anddecrescendo;

Fig. 4 is a diagram illustrating tone compensating action; and I Fig. 5is a diagram of the variation in volume range of a selection onrecording and reproducing.

In the following description and in the claims, various details will beidentified by specific names for convenience, but they are intended tobe as limiting control generic in their application as the art willpermit.

In the drawings accompanying and forming part of this'specification,certain specific disclosure of the invention is made for purposes ofexplanation, but it will be understood that the details may be modifiedin various respects without departure from the broad aspect of theinvention.

Referring now to the drawings, and more particularly to Fig. l, a soundreproducing system is shown having an electro-mechanical pick-up deviceI arranged to pick up sound energy from a mechanical sound record and aphotoelectric cell 2 arranged to receive sound energy variations from aphotographic sound record.

These pick-up devices opposite poles of double-pole, double-throw switch3 adapted to connect either device to the input circuit of potentiometer4L Potentiometer 4 is provided with sliding contacts 5 arranged to varythe input of energy to primary winding 5 of transformer 1.-

Secondary winding 8 of transformer I is connected in the input circuitof vacuum tube amplifier 9. The output circuit of amplifier 9 comprisesthe primary winding 10 of audio-frequency transformer II and also theinput circuit l3 of the control channel. I

Secondary winding I2 of transformer l I is connected in the inputcircuit of amplifier l4 comprising tetrode vacuum tubes l5 and I6. Tubel5 contains cathode l1, anode l9 and two grid or control electrod es 2|and 23. Tube l6 contains cathode l8, anode 29 and two grid or controlelectrodes 22 and 24.

.Control electrodes 2| and 22 are located nearest the cathodes and areconnected to the output l and 2 are connected to transformer 29.

' is determined by thebiasing of the control elec-' I circuit 2| of thecontrol channel, as will be more fully described later.

Control electrodes 23 and 24 are located nearest the anodes and may,ifdesired, comprise r shield grids for the anodes. These controlelectrodes are connected to the ends of secondary winding l2 oftransformer H. with biasing battery 26 connected to the midpointthereof, thus provides part of the :sound energy input circuit fornecting the input circuit amplifier II. By conto the shield electrodesdistortion will be kept at a minimum. It is quite feasible, however, andwithin the scope of the present invention, to con-' nect the signalinput circuit to grids 2| and 22 and the control circuit to grids 23 and24.

The anode or output circuit of amplifier I 4 com-' prises primarywinding 28 of audio-frequency I The mid-point of winding 28 is connectedto plate biasing battery 27.

Transformer 29 is designed with certain desired irequencycharacteristics whereby it wiil act as a band-suppression filterat lowsignal volumes to reduce the transmission of the intermediate frequencyregisters to a greater extent than the high and low registers. however,the transformerhas substantially uniform efficiency throughout theaudible frequency range.

The correct design for transformer 28*may be determined by well knownand standard methods and will depend upon the characteristics of theamplifier tubes II and It with which it is to be connected. thetransformer windings are tuned to discriminate against intermediatefrequencies. With a tuned circuit of this type it is well known that thefrequency discriminating effect will be a direct function of the circuitimpedance. In the present instance the impedance will be determined bythe amplifier resistance which, in turn,

trodes.

When the tube impedance is high the input load for the transformerprimary 28 is negligible. Under these conditions the frequencydiscrimination is most effective and the transmissionof high and lowfrequencies is accentuated due to the tuning of the transformer. Whenthe tube impedance is lowered the input load for primary 23 will beraised and the frequency discrimination will decrease untilsubstantially uniform response over the entire frequency range will beobtained for high signal volumes. A

Secondary winding 30 of transformer 29 is connected in the input circuitof push-pull amplifier 34 comprising vacuum tubes 35 and. In the Presentembodiment the nected to the inner grids which are adjacent to thecathodes. The shield grids (nearer tothe anodes) are connected throughbiasing battery 3| to the cathodes.

The outputcircuit of amplifier 34 includes the primary 33 of outputtransformer 33. The midpoint of winding 33 is connected to the cathodethrough pla e biasing battery 32 and battery 3|. Thesecondary ll ofoutput transformer 33 is connected through potentiometer 4| to themoving coil of loud-speaker 43. The fixed coil of the speaker issupplied with current by means of battery I. Potentiometer; II isprovided with input circuit is consliding contacts 42 for varying thevolume of the anodes of a pair oi' two element At high signalintensities, envelope of the rectified In any event, certain sectionsofnected to sliding contacts "to the anode or plate of i3 is connectedto the input of amplifier tube I in the control circuit by acondenser-resistance coupling comprising condenser Ii and resistance32.- The output of amplifier. Ellis connected through transformer 53 topush-pull ampliiler 54 comprising vacuum tubes "and 58.

The output of push-pull a the primary I! of transformer 53. Secondarywinding '3 of transformer 53 is connected to the thermionic tubes BI and82, together comprising a full-wave rectifler Bil.

The center of secondary winding 59 and the cathodes of tubes BI and 62-are connected to opposite ends of resistance 63, which will hereafter hecalled the control resistance. Current from rectifier 60 will passthrough control resistance 63 rendering the upper end of the resistance(as seen in Figure 1) positive with respect to the lower end.

Connected in parallel with resistance 63-is a condenser 64 which servesas a filter for the audio-frequency fluctuations in the rectified cur-Circuit amplifier il includes rent from rectifier 60. The currentthrough resistance 63 will thus'be averaged to represent-the currentsor, in other words, the intensity of the audio-frequency signal.

Control resistance 63 comprises part of the output circuit 25 of thecontrol channel.- The entire output circuit'includes the followingelements in series: control grids 2| and 22 of amplifier l4, resistance65, control resistance 33, biasing potentiometer 3'! -andcathodes i1 and13 of amplifier ll. Biasing battery 66 is connected in parallel withpotentiometer 6". to furnish a potential gradient across thepotentiometer.- The normal or initial bias on control grids 2| and 22may be adjusted by moving sliding, contact Bl along potentiometer 61.

Discharge devices 69 and III are connected across control resistance 63and serve to limit the potential which can be developed across thisresistance. The cathodes of these tubes are conand 14 adapted to slideon potentiometer resistances l2 and II, respectively, both of which areconnected across biasing battery 66 in the same manner as potentiometer61.

The anodes of tubes 68 nected to the 63.

and III are both conpositive end of control resistance The grid of tube33 is connected directly the tube. Thefgrid. of tube Hi, however,is-connected to contact '13 arranged to slide on control resistance 63.

. A variable delay arrangement is also associated with output circuit 25of the 'controlchannel. Thisdelay arrangement comprises resistance 65,connected in output circuit 25 and variable condenser 13 connectedacross both resistances 63 and 35. These elements form acondenser-resistance discharge device the inertial characteristics ofwhich may be vatied by adjusting variable condenser 16.

Connected in parallel with resistance 85 are two three-element vacuumtubes 11 and II. These tubes are connected in parallel opposingrelation, that is, the anode of tube I1 and the cathode of tube 18 areboth connected to one end of resistance 65 and-the cathode of 11 andanode of II to theother end. '-'I'he grid elements of tubes l1 and 13are connected, respec tively, to sliding contacts 13 and 80 arranged toslide on resistance 65. Thus the grid biases of the tubes may be variedindependently to regu- 1m the individual internal impedance of thesetubes; The inertial or delay action may thus be separately regulated iorbuild-up and decay of shown) dynamic expansion will take place. If

the switch is thrown to the up position the control resistance isshunted and the sound amplifier connected to operate without dynamicexpansion at a gain ratio which will be determined by the setting ofbiasing contact ll assoclated with potentiometer 12. If the switch 8| isthrown to the "down" position, the control resistance is shunted toeliminate dynamic expansion, and the gain ratio oi the amplifier will bedetermined by the setting of biasing contact [4 associated withpotentiometer 15.

Operation In the operation of the system in reproducing a selection froma mechanical sound record, pickup device i is placed in operativerelation to the. moving sound track of the record, or if a. photographicrecording is used, cell 2 is arranged to receive light variations fromthe moving photo.- graphic sound track. The chosen pick-up deviceisconnected by switch 3 to potentiometer 4. Contacts 5 are adjusted toallow a desired volume of input energy to reach transformer I.

The sound energy passes through transformer I and is amplified byamplifier 8. Part of the amplified energy passes through transformer IIand is further amplified by push-pull amplifier l4 whose gain ratio issimultaneously varied by the control grids 2i and 22.

The control of the gain ratio is brought about as follows: Part of thesignal energy which has been amplified by vacuum tube 9 is fed throughcontrol input circuit l3 and the condenser-resistance coupling 5|, 52 toamplifier 50. The output energy from amplifier 50 passes throughtransformer 53 and is further amplified by push-. pull amplifier 54. Theoutput of 54 passes through transformer 58.

The resulting alternating current is rectified by full wave rectifier 60to yield a direct current which passes through control resistance 63from the cathodes of rectifier 60- to the center of sec ondary winding59 of the transformer. It will be obvious that the average strength ofthis direct current over any limited interval will be directlyproportional to the strength of the input signal. Condenser 64 serves tofilter out the audio-frequency fluctuations of the rectified current.The current through resistance 63 will then represent an envelope of therectified cur-' rent impulses.

The potential across resistance 53 will accordingly be varied-along withthe strength of the rectified current,'the upper end of the resistance(as shown in Fig.1) being made positive with frequency signals. By suchcontrol action the gain ratio of amplifier l4. may in some instances bevaried over a range of 30 decibels between low and high signal volumes.By adjusting contact 68 on potentiometer I! the initial bias on thecontrol electrodes and hence the initial amplification setting of theamplifier tubes l5 and I6 may be adjusted to any desired value.

Discharge tubes 69 and Ill serve to limit the change in bias which thecontrol circuit may produceon the control electrodes. Tube 8.9 preventsthe control channel from causing more than a predetermined maximumchange in the bias oi control electrodes 2| and 22. controls thepercentage change in bias which the control channel may bring about forany signal volume. r

The action of the limiter tubes may be illustrated by referring to Fig.2 in conjunction with Fig. 1. 'With the settingof the potentiometer.

contacts 68, H and 14, as shown (Fig. 1) the anodes of limiting devices69 and ill will be negatively biased with respect to their cathodes aslong as no current is passing through resistance 63. When rectifiedsignal current passes through resistance 53, thereby causing a potentialdrop in this resistance, the anodes of devices 69 and 10 will be madeless negative and will occasionally become positive with respect totheir cathodes when the higher signal volumes are reached. The anode ofdevice 10, for example, will become positive when the potential dropacross resistance 63, determined by the volume of input signal, hasreached a value sufiicient to overcome the initial'bias on the anode.This is repre- 'sented by point I08 in Fig. 2. When this potential valueis reached a discharge will begin between the anode and cathode ofdevice 10, this device thereby serving to shunt a portion of the currentreceived from rectifier 50.

While the anode or plate bias of tube 10 is varied by the change intotal potential drop across control resistance 63 the grid bias of thistube may be varied as any fraction of the potential drop across thecontrol resistance. This fraction is determined by setting slidingcontact 13 to'any desired point on the control resistance. Accordingly,device ill will have a variable impedance the value of which isdetermined by the potential drop across control resistance 53.

Thus by setting contacts 13 and 14 the potential drop across controlresistance 63, and hence the expansion characteristics of the amplifier.

' may be adjusted to correspond to any one of sev- Tube ll.

that the potential drop across resistance 63 has the values indicated bycurve ill] for various values of input signal. In the reproduction ofmusic the potential drop may preferably have the values shown by curvelil.

Should the input energy approach very high values, there is danger thatthe bias on control electrodes 2! and 22 may be reduced to an undulysmall negative value thereby causing .distortion of the signal; Suchexcessive changes in the bins are prevented by discharge tube 69. Whenthe potential drop across control resistance 63 increases to a pointwhere it is suflicient to overcome the initial bias of tube 69(indicated by point Hi9, Figure 2) this tube will begin to discharge.Since the grid element of tube 59 is connected directly to the anode,the imped- 7 contact Il tential drop across control resistance 83 willthus be held to a substantially constant value (indicated by horizontalline H2 in Figure 2) and the gain ratio of the amplifier will, ofcourse, also be constant.

The efiect of the limiter tubes in shaping the gain characteristics willbe more evident by comparing the above mentioned curves with curve 3(Figure 2) which represents the variation in control potential forvarious values of input signal volume when no limiting devices are used.

During reproduction sudden changes in signal volume will often occur, ason rapid crescendo and rapid decrescendo. By variously delaying thechange in gainratio of the amplifier when these changes in signal occura more pleasing and natural efi'ect will be obtained. The variable delaycircuit permits separate adjustment of the delay for crescendo anddecrescendo.

If a sudden increase in signal volume'occurs during reproduction, as onrapid crescendo, the potential drop across control resistance 63 willincrease immediately. The change in potential applied to the controlelectrodes will be delayed, however, due to the action of the variabledelay circuit. The length of delay will depend on the capacity ofcondenser 18 and the impedance of tube Il. Likewise, onsudden decreaseof signal volume, as on rapid decrescendo, the potential drop acrossresistance 63 will decrease with the decrease in volume but the changein potential 40 on the control electrodes will be delayed. The length ofdelay will depend on the capacity of condenser 16 and the impedance oftube 18. Adjustment of the capacity oi condenser 18 will accordinglyvary the delay for both crescendo and decrescendo.

' dered..

Fig. 3 illustrates diagrammatically the delay on crescendo anddecrescendo. the curves represent-- ing the change in potential acrosscondenser 16 on build up and decay of signal. It is obvious that thegain ratio of amplifier II can be represented bysimilar curves.

. Curve I" shows the charging rate of the condenser for rapid attack orbuild up of amplification. In this instance, contact 19 will be set nearthe lower end of resistance I! (as shown in Fig. 1) and the impedance oftube 11 will be low. Curve 7 III shows the charging rate of thecondenser for slow attack or-build up of amplification. Thetimelndicated on the graph by the distance between points I]! and I"represents the variation in delay which may be achieved by adjusting Inaddition, separate adjustment of delay on Curves I04 and I05 representthe ratesof dis-- charge of condenser 16 on signal volumedecay ordecrescendo. Curve I shows the discharging rate of the condenser whencontact "is set'fcr rapid decay of the signal (1. e. set near the upperend of resistance 65, as shown in Fig.-l)-."-1"'"Curve Hi5 shows thedischarging rate of the condenser forslow decay or falling oil ofamplification. In

this case, the contact 80 would beset near the lower end of resistance*as shown inaFig. 1. The time represented on the graph by the distancebetween points 106 and ill-1 represents the variation in delay ondescrescendo which may be achieved by'adiusting contact 80.

It will usually be found desirable with high quality-recordings toadjust the variable delay circuit to cause rapid attack and slow .decayof the gain ratio with changes in signal volume. In some instances othersettings may be found usefuland can readily be obtained by adjustingcondenser 16 and contacts 19 and B0.

In case it is desired to operate the amplifier without dynamicamplification, i. e. without change of the gain ratio of the amplifierduring the rendering of the selection, switch Cl is moved to the "up ordown" position (as shown on Fig. 1). If the switch is moved to thefup"position the control electrodes are given a steady bias the value ofwhich is determined by the position of contact H on potentiometer l2. 1If switch II .is thrown to the down position, the control electrodes aregiven a steady bias determined by the position of contact 14 onpotentiometer II. It will thus be seen that the amplifier may be set tooperate at any one of a plurality of steady gain ratios by simplyoperating switch 8|.

Switch 8|, in combination with potentiometer contacts H and 14, providesa convenient means for testing and adjusting the bias on the limitingdevices 69 and 10. Thus by moving the switch 8| to the up position andthen adjusting con-' tact H to give amplifier H the maximum gain ratioat which it is desired to operate, and then returning the switch to themiddle position, tube 69 will be given a bias which will cause it toshunt the control resistance when the said maximum value of gain ratiois reached. If switch Ii is thrown to the down position, contact I4 maybe manipulated. In this case the resulting gain ratio of amplifier Mwill indicate the point at which tube ill will start to shunt thecontrol resistance when the switch is again set for dynamic expansion.

With the switch 8| set for dynamic expansion the audios-frequency signalpassing through transformer II will be variably amplified by amplifierll under control of the control channel and will then be fed-throughtransformer 29 to amplifier 34. speaker 43 throughtransformer 39 andpotentiometer II. The volume 01' output may be adjusted by movingcontacts 42 associated with potentiometer l I.

The separate control electrodes in amplifier ll,

by isolating the control channel from the audiofrequency channel,prevent distortion of the signal by any audio-frequencies which are notfiltered out of the control current bycondenser 84. Likewise, any evenharmonics of the audio-frequency signal introduced into the outputcircuit of amplifier ll by the control circuit, will be suppressed dueto the push-pull arrangement of tubes l5 and I 8.

Transformer II will exert a frequency selective eflcct on the signaliasbrought out'above). i. e.

Here it is further amplified and fedto loud I for low. signal values thehigh and low frequency registers willbe accentuated. For high volumesall frequencies will be transmitted with substantially equal efilciency.It is well known that when the volume of a sound is decreased thesensitivity of the human ear to the low and high frequency componentsfalls of! morerapidly than the sensitivity to the intermediatefrequencies. The present frequency selective arrangement compensates forthis peculiarity or defect of the ear, whereby selections reproduced atlower volume levels than the original rendition will pro-- duceproportionally the same effect on the ear of the listener for allfrequency registers. This effect is illustrated diagrammatically in Fig.4.

If a highly intensity sound were produced having all frequencies presentin equal intensities throughout the audible frequency range, theintensity at all frequencies would be represented on-the diagram (Fig.4) by a horizontal line A, which gives the actual intensity of the soundin decibels for any frequency. The effect on the ear of a listener, orthe apparent intensity at different'frequencies for this sound, willthen be represented by curve B. If the sound were reproduced with thevolume uniformly lowered through the frequency range, the outputintensity of such a perfect" reproducer would be represented byhorizontal line C.

For this decreased intensity, however, the high er will be affected to alesser extent by the low volume high andlow frequency tones than by thelow volume intermediate frequency tones, as indicated in curve D.

I The present reproducing circuit is arranged to compensate for thispeculiarity of the human ear by reproducing the high and low frequencieswith less reduction from the original intensities than the intermediatefrequencies. This is illustrated in curve E, which represents the actualintensity on a'eproduction by the present circuit, of a sound having anoriginal frequency distribution represented by curve A. j

The effect on the ear of the low volume sound represented in curve E,would then be equal for all frequencies, as indicated by horizontal lineF, which coincides with horizontal line C on the frequency distributiondiagram. The original high intensity sound is thus reproduced at a lowerintensity while preserving substantially the same effect of allfrequencies on the car as would be 0 be made nearly independent ofsignal volume.

produced by the original sound.

By adjusting input potentiometer 4, the frequency control action may bereduced or increased. By reducing the input, for example, the frequencycharacteristics of transformer 29 may The output volume,rof course, maybe kept the same (when the input is reduced) by adjusting" ao,4aa Ipensatingmeans are provided whereby sounds may be reproduced in a normalmanner'for all signal volumes. Various other features have been providedwhereby the operation of'the system in controlling sound energy has beenimproved.

It will be recognized that in the recording of selections on soundrecords of either mechanical or photographic type there are certainupper and .lower limits of volume above or below which tra. For purposesof comparison the flducial or v reference point, marked 0 decibels onthe scale,

' has been taken to represent a volume level equal in electrical unitsto the power dissipated in a 500 ohm resistance having 2.5 volts acrossits terminals. The loudest sound reached by the orchestra may then havea value of 35 decibels above the reference level and the softest soundmay be substantially decibels below that reference level,--

as shown. This minus 80 decibel point will correspond substantially tothe threshold of hearing.

Even with modern high quality recording, the soft notes must beamplified before recording in order to bring them above the volume levelof the background noises, such as scratch introduced by irregularitiesin the record. The loudest sounds on the other hand, must be attenuatedin intensity in order to avoid overcutting of the sound track. Forsatisfactory high quality recording, i

the sounds might preferably be contracted to a volume range between plus20 and minus 35 decibels as indicated in the area between A and B (Fig.5). The sound may then be satisfactorily recorded.

On reproduction it is desirable to simulate the volumes of soundproduced by the original sound source and hence the volume range maypreferably be expanded as indicated in the area between C and D of Fig.5. It will be noted that the softer sounds are actually decreased involume on reproduction and thus needle scratch and background noises aresuppressed or eliminated.

The present circuit presents an ideal means for reproducing andexpanding the sound selection. It will be noted, however, thatapplicant's circuit is likewise adaptable for recording ,the sounds inwhich case the circuit will be arranged to compress the volume range asindicated in the area between A and B (Fig. 5).

While certain novel features, of the invention have been disclosed andare pointed'out in the annexed claims, it will be understood thatvarious omissions, substitutions and changes may be made by thoseskilled in the art without departing from the spirit of the invention.

What is 'claimedis:

1. In an energy translating system for use with a sound record a firsttranslating device,

7 a second translating device fed thereby and means between said devicesfor discriminating against intermediate sound frequencies when thesignal energy is low and for passingall sound frequencies withsubstantially equal efficiency when said energy is high. I

2. In a sound record system, a pick-up device, a filter network'fedthereby, said network being adapted to reduce the transmission ofintermedihigh signal intensities and a translating device ted by saidnetwork.

3. In a sound translating system for use with a sound record, a firsttranslating device, an

' amplifier fed thereby, a transformerTed by said amplifier, saidtransformer discriminating against intermediate sound frequencies whensaid amplifier impedance is high and passing all sound frequencies withsubstantially equal efliciency when said amplifier impedance is low, anda second translating device fed by said transformer.

4. In a sound reproducing system for use with a sound record, a pick-updevice, an output device ted thereby, means between said devices fordiscriminating against intermediate frequencies when the signal volumeis low and for passing all frequencies with substantially equalefilciency when the signal volume is high.

5. The method of improving the characteristics of sound on reproductionfrom a sound record which comprises reproducing the high and low soundfrequency registers in higher ratio 01' volume to the intermediatefrequencies for low sound volumes than 101' high sound volumes.

- 6. In a sound translating system for use with a sound record, a firsttranslating device, an amplifier ted thereby, a second translatingdevice ted by said amplifier, a control circuit for varying the gainratio of said amplifier and a governing device for varying theeffectiveness of the control exercised by said control circuit over asubstantial range of signal volumes.

7. In a sound translating system for use with a sound record, a firsttranslating device, an amplifier ted thereby and a second translatingdevice ied by said amplifier, a control circuit fed by said firsttranslating device for changing the gain ratio 01' said amplifieraccording to a function of the energy received by said control circuitand a device ,ior governing the effect 01 said control circuit wherebysaid iunctlon may be varied. I j

8. In a sound translating system for use with a sound record, a firsttranslating device, an amplifier ted thereby and having a controlelectrode, a second translating device fed by said amplifier,

means for varying the gain ratio of said amplifier ratio or saidamplifier, a. first limiting device for determining the efi'ectivenessoi. said control circuit overa continuous ,range of values, and asecondlimitingdevice for determining the maximum' eflectiveness of saidcontrol circuit.

10. The method of predeterminin'g the maximum gain ratio of a dynamicamplifier used in. a sound translating system for a sound record andhaving a limiting device associated therewith which comprises adjustingthe setting of said amplifier to operate at said maximum gain ratio andcausing said setting to determine the point above which said limitingdevice will become cn'ective.

- 11. In a sound translating system for use with a sound record. a firsttranslating device, an ammeans ior. delaying the action or said controlplifler ted thereby, a second translating device fed by said amplifierand a control circuit for varying the gain ratio of said amplifier, saidcontrol circuit having a timing arrangement comprising a variableimpedance and a variable capacitance for determining its rate of action.

12. In a sound translating system for use with a sound record, a firsttranslating device, an amplifier fed thereby and having a controlelectrade, a second translating device fed by said amplifier, means forvarying the gain ratio of said amplifier comprising a control circuithaving a biasing impedance for determining the bias on said controlelectrode, said control circuit having means for delaying the controlaction, said means comprising a second impedance and a I variablecondenser.

control action and comprising a second impedance in series with saidbiasing impedance'and means for varying the value of said impedance.

14. In a sound translating system for use with a sound record, a firsttranslating device, an amplifier fed thereby and having a controlelectrode,

a second translating device fed by said amplifier, f

a control circuit for varying the gain ratio of said amplifier fed bysaid first translating device and having a biasing resistance in thecircuit of said control electrode, a second resistance in series withsaid biasing resistance and said electrode, a capacitance in shunt withsaid two resistances, an impedance in shunt with said second resistanceand means for varying the value thereof.

, I 15. In a sound translating system for use with a sound record, afirst translating device, a second translating device fed thereby, acontrol electrode for varying the eificiency with which audio-frequencyenergy is transmitted from said first to said second device according tothe potential applied thereto, means ior varying the potential appliedto said electrode comprising a circuit element having a variablepotential gradient across its terminals, an impedance in series withsaid circuit element, a capacitance in shunt with said element and saidimpedance and means for ad-- lusting the eflective'value of saidimpedance.

16. In a system for reproducing sound from a sound record, a pick-updevice, an amplifier ted thereby and having a control electrode, atranslating device fed by said amplifier, a control circuit fed by saidpick-up device for varying the -.'gain ratio of said amplifier, a timingcircuit connected between said control circuit and said controlelectrode, said timing circuit comprising 'a.

capacitance and a resistance in the charging and discharging circuit ofsaid capacitance, a pair of unidirectional discharge devices oppositelyconnected in shunt with said resistance and means for separatelyadjusting the impedance of 89.1 discharge devices- 1 17. In a system forreproducing sound from a sound record, a pick-up device, an amplifierfed thereby and an output device red by said amplifier, a controlcircuit ted by said pick-up device for varying the gain ratio of saidamplifier according to a moving average of the signalenergy.

circuit and means separately varying the length of delay for crescendoand for decrescendo.

18.; In an energy translating system .for use with a sound record, afirst translating device, a transformer fed thereby, means renderingsaid transformer normally selective to predetermined frequency bands,means rendering said transformer non-selectively responsive to highsignal volumes and a second translating device fed by said transformer.x

19. In a sound reproducing system for use with a sound record, a pick-updevice, an output device i'ed thereby, and means between said devicesforselectively discriminating against a predetermined band of recordedsound frequencies when the sound volume is low and for passing allrecorded sound frequencies with substantially equal efficiency when saidvolume is high.

20. In a sound translating system for use with a sound record, a firsttranslating device, an amplifier fed thereby and a second translatingdevice fed by said amplifier, a control circuit fed by said firsttranslating device for varying the gain ratio of said amplifierresponsive to the energy received-from said device, and a variablelimiter for varying the degree of control exercised bysaid controlcircuit responsive to the energy received by said control circuit.

21. In a sound translating system foruse with a sound record, a firsttranslating device, an amplifier ted thereby and having a controlelectrode, a second translating device fed by said amplifier, means tovary the gain ratio of said amplifier comprising a control circuitincluding a biasing resistance for determining the bias on said controlelectrode, a discharge device connected in .shunt with said resistance,a grid electrode for determining the impedance'of said discharge device,and means for controlling the bias on said grid electrode responsive tothe potential drop across said biasing resistance.

.22. In a' sound translating system for use with a sound record, a firsttranslating device, a secr ond translating device fed thereby, a controlelecacteristics or the amp trode for vafi'lng the efiiciency with whichaudioi'requency energy is'transmitted from said first to said seconddevice according to the potential applied thereto, means for varying thepotential applied to said electrode comprising a circuit element havinga variable potential gradient across its-terminals, an impedance inseries with said circuit element, a capacitancein shunt with saidelement and said impedance, and means for adlusting said impedance'tohave difl'erent values for charge and for discharge of saidcapacitance. 23. In a sound record system, a pick-up device, anamplifier red thereby and a translating device fed by said amplifier,said amplifier having a signal input "grid and having a negativelybiased control grid for varyin the amplification charer accordancewith-the signal strength.

24. In an energy "tran lating system fdr' use with a :sound record. atranslating device; a

space discharge device havinga grid connected to receive audio-frequencyvariations from said first translating device and a separate, negativelybiased grid connected tovary the transmission characteristics of saiddischarge device in accordance with the energy value of saidaudio-irequency variations and a second translating device fed by saiddischarge device.

25. Inc. sound reproducing system for use with a sound record, a pick-updevice, an amplifier fed thereby and a translating device fed by saidamplifiensaid amplifier comprising a space discharge device having ananode, a first control electrode fed by audio-frequency variations fromsaid pick-up device and -a second negatively biased control electrodefed by sub-audio-frequency variations from said pick-up device forcontrolling the gain characteristics of said amplifier.

26. In an audio-frequency amplifier system, a gain control stagecomprising a vacuum tube having an anode, a cathode, a first controlelectrode and a second negatively biased control electrode, means forapplying audio-frequency variations to said first control electrode andmeans for applying sub-audio control variations to said secondnegatively biased control electrode.

27. In an audio amplifier of the type including a space discharge devicehaving a cathode, a, signal input electrode, a signal output anode andat least one auxiliary negatively biased control electrode, automaticmeans actuated in accordance with an increase in average audio inputintensity grid, an output electrode and at least one negatively'biasedcontrol electrode functioning as a gain control electrode, said signalgrid and cathode being connected to said input circuit, an

audio range expansion network comprising an audio signal amplifierhaving an input circuit arranged to'have audio signals impressedthereon, means for developing a direct current potential irom rectifiedaudio signals and means impressing said direct potential on saidnegatively biased electrode in a polarity sense such that the amplifiergain increases with increase in audio signal intensity. a

29. The method of obtaining gain variations in a signal amplifierforming part of a sound energy translating system and having an,anodeand a plurality of control electrodes, at least one of which isnegatively bi'asedj which comprises introducing sub-audio controlvariations into said system through said negatively biased controlelectrode and introducing audio-frequency variations into said systemthrough the other of said control electrodes.

ARTHUR Plasma.

